. The main goal of the proposed research is to determine the structure of the principal neutralizing determinant (PND) of HIV-1 complexed with specific monoclonal antibodies. A secondary goal is to crystallize the surface glycoprotein of HIV-1, gp120, as a complex with neutralizing monoclonal antibodies. The specific goals of this proposal are to determine the structures of the PND from different isolates, in particular MN and IIIB, complexed with their specific neutralizing antibodies. The Fabs to be studied are 50.1 (MN specific), 0.5 beta (111 B specific) and the more broadly neutralizing 59.1 (MN, IIIB, WMJ2, SF2, specific). It is the intent to determine the conformation of the PND bound to each antibody and hence to understand how different antibodies recognize different isolates. Valuable information will be obtained not only of the structure and function of gp120 but also the structure of the PND in the intact antigen. The investigator believes that the structural information obtained from all those studies should be invaluable in the design of immunogens constrained to adopt their biologically relevant conformations as is currently in progress for another peptide immunogen in the principal investigator's laboratory. The structures will be determined by x-ray crystallography using molecular replacement (MR) or multiple isomorphous replacement. A brief summary of the specific aims is: 1). Crystal structure of Fab 50.1 + PND peptides, 2). Crystal structure of Fab 0.5 beta + PND peptides. 3). Crystal structure of Fab 59.1 + PND peptides, and 4).Crystallization of gp120-Fab complexes.